One day last year, a picture showed up on Paul Kelly’s door. Kelly, a biologist at Salem State University, thought it was an image of calcified bone material taken by his officemate using a scanning electron microscope, a device that creates images at the atomic scale.

It turned out to be a photo not of something very small, but of something very big — sand dunes in the Sahara, taken by satellites. It belonged to Stephen Young, a geographer who worked a few floors away and who had noticed that Kelly’s microscopic images sometimes exhibited the same patterns as his own macroscopic ones.

“We’re both interested in spatial patterns,” Young told Weather.com. “I’m just interested in them at the macro scale and Paul’s interested in them at the micro scale.”

Kelly, for example, sometimes works with other geologists in the building, looking at thin slices of minerals under the electron microscope. When minerals shatter, they follow specific breaking patterns that stay consistent even when scaled up.

“It’s the same at the microscope and macroscopic levels,” Kelly told Weather.com. “The way minerals shatter and the way rocks shatter is going to be the same no matter what the scale.”

The scope of Young’s images ranges from about one mile to about 3,000 miles; Kelly’s are generally a millimeter to a micrometer. At the closest, Young said, the sizes in their images are different by a magnitude of a million.

Still, it can be hard to tell the pictures apart. Kelly and Young decided to gather a collection of their work and display them in a “Macro or Micro?” exhibition at Salem State’s Winfisky Gallery and the Traina Center for the Arts at Clark University. Visitors often guessed wrong, including trained scientists.

The exhibit also stirred interest in some of the university’s art students, who are going to use Kelly’s microscope to make artwork.

Can you tell which is which? Click through the slideshow above to guess whether each image is “macro” or “micro.”